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Float Charging Performance of Lithium Ion Batteries with LiFePO4 Cathode
Abstract
We performed a long-term test to determine the float charging durability of prismatic cells with Mn containing LiFePO4-based cathode material at various temperatures. The cell maintained more than 70% of its initial discharge capacity after float charging at 4.0 V and 25 degrees C for 24 months. In contrast, the capacity decreased rapidly in a few months for a cell float charged at 55 degrees C. To investigate the capacity fading mechanisms, we analyzed the cathode and the anode sheets removed from the prismatic cell used for the float charging test at 55 degrees C with electrochemical, XRD and EPMA techniques.
... On the other hand, according to the experiment in reference [15], the LiFePO 4 cathode battery retained 70% of its initial discharge capacity after a 24-month float charge test. The capacity reduction rate increased with increasing temperature, and the capacity dropped to 60% of the initial value after one month of float charging test performed at 55 • C. Therefore, both the float charge voltage and float charge time must be taken into account when determining the required capacity of a lithium-ion stationary battery. ...
... On the other hand, according to the experiment in reference [15], the LiFePO4 cathode battery retained 70% of its initial discharge capacity after a 24-month float charge test. The capacity reduction rate increased with increasing temperature, and the capacity dropped to 60% of the initial value after one month of float charging test performed at 55 °C. ...
Lead-acid batteries are currently the most popular for direct current (DC) power in power plants. They are also the most widely used electric energy storage device but too much space is needed to increase energy storage. Lithium-ion batteries have a higher energy density, allowing them to store more energy than other types of batteries. The purpose of this paper is to elaborate on the factors affecting the capacity design of lithium-ion stationary batteries. Factors that need to be considered in calculating the capacity of stationary lithium-ion batteries are investigated and reviewed, and based on the results, a method of calculating capacity of stationary lithium-ion batteries for industrial use is proposed. In addition, the capacity and area required for replacing the lead-acid batteries for nuclear power plants with lithium-ion batteries are reviewed as part of this case study.
Nanocomposites of LiFePO4 and conductive carbon were prepared by two different methods which lead to enhanced electrochemical accessibility of the Fe redox centers in this insulating material. Method A employs a composite of the phosphate with a carbon xerogel formed from a resorcinol-formaldehyde precursor; method B uses surface-oxidized carbon particles to act as a nucleating agent for phosphate growth. Both particle size minimization and intimate carbon contact are necessary to optimize electrochemical performance. Although both methods succeed for the first criteria, the latter is best achieved with method A, affording excellent characteristics in room temperature, liquid electrolyte cells. The resultant LiFePO4 /C composite achieves 90% theoretical capacity at C/2, with very good rate capability and excellent stability. (C) 2001 The Electrochemical Society.
LiFePO4 powders were synthesized under various conditions and the performance of the cathodes was evaluated using coin cells. The samples were characterized by X-ray diffraction, scanning electron microscope observations, Brunauer, Emmett, and Teller surface area measurements, particle-size distribution measurements, and Mossbauer spectroscopy. Ab initio calculation was used to confirm the experimental redox potentials and Mossbauer parameters. The choice of a moderate sintering temperature (500 degreesC < T < 600 degreesC) and a homogeneous precursor enabled nearly perfect utilization of >95% of the 170 mAh/g theoretical capacity at room temperature. There are two main obstacles to achieving optimum charge/discharge performance of LiFePO4: (i) undesirable particle growth at T > 600 degreesC and (ii) the presence of a noncrystalline residual Fe3+ phase at T < 500 C.
We fabricated prismatic lithium-ion cells with a
-based cathode material and examined the long-term cyclability and high thermal stability of the
in the cells to support the application of this material in large cells. The cells exhibited excellent long-term cyclability.
In a high rate charge-discharge test, more than 6000 cycles were completed without any serious reduction in capacity. A cathode
electrode sheet incorporating
-based material taken from the cell after a test consisting of more than 8000 cycles exhibited little deterioration compared
with an electrode sheet obtained from an unused cell. The thermal stability of the
in the cells was examined with a heating test at 180°C, a nail penetration test, and a crush test. Although the cell case
swelled slightly in the heating test, the cells did not smoke or catch fire in any of the safety tests. The cathode electrode
taken from the prismatic cell used in the 180°C heating test had a similar capacity and discharge profile to a cathode taken
from an untested cell.
The rechargeable lithium-ion battery has been extensively used in mobile communication and portable instruments due to its many advantages, such as high volumetric and gravimetric energy density and low self-discharge rate. In addition, it is the most promising candidate as the power source for (hybrid) electric vehicles and stationary energy storage. For these applications in durable equipment, the long-term cycling and storage behaviour becomes of increasing interest. In this paper, the mechanisms of lithium-ion battery ageing are reviewed and evaluated.
Low-cost lithium ion batteries with LiFePO4 and natural graphite were cycled in 1 M LiBF4+EC/DEC at 100% depth of discharge and 25 °C in order to investigate cycle performance and diagnostics for capacity fading. The 12 cm2 pouch cell showed 65% of capacity retention at 5C compared to that at C/25. The cell showed 80% of initial capacity after 80 cycles and its capacity fade rate was 11.3 μA h/cycle during constant C/2 cycling. In hybrid pulse power characterization, the discharge resistance of this cell was higher than commercial graphite/LiCoO2 cell because of low lithium diffusivity in LiFePO4. Slow rate cycling in pouch full cell showed almost 40% of capacity fade after 100 cycles. However, the 100-cycled cathode and anode did not show any capacity fading in half-cell test after disassembling full cell, suggesting that capacity fade in the full cell is caused by loss of cycleable Li.
Amorphous LiFePO4 was obtained by lithiation of FePO4 synthesized by spontaneous precipitation from equimolar aqueous solutions of Fe(NH4)2(SO4)2·6H2O and NH4H2PO4, using hydrogen peroxide as oxidizing agent. Nano-crystalline LiFePO4 was obtained by heating amorphous nano-sized LiFePO4 for different periods of time. The materials were characterized by TG, DTA, X-ray powder diffraction, scanning electron microscopy (SEM) and BET. All materials showed very good electrochemical performance in terms of energy and power density. Upon cycling, a capacity fading affected the materials, thus reducing the electrochemical performance. Nevertheless, the fading decreased upon cycling and after the 200th cycle the cell was able to cycle for more than 500 cycles without further fading.
We investigated the effect of cell temperature on the electrochemical reaction behavior of LiFePO4. We evaluated its electrochemical characteristics by using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Both the electron transfer activity and the lithium ion diffusion rate in the LiFePO4 electrode increased as we increased the operating temperature. The apparent activation energy for the lithium diffusion was more than twice that for charge transfer. This result supports the idea that the large discharge capacity of LiFePO4 at elevated temperatures is mainly caused by the high lithium diffusion rate in the LiFePO4 particles, which makes it possible to utilize more material near the center of the particles. We also evaluated the thermal stability of LiFePO4 by differential scanning calorimetry (DSC). Charged LiFePO4 in contact with an electrolyte solution exhibited no obvious endothermic or exothermic reaction below 340 °C. This means that LiFePO4 has higher thermal stability than such other cathode materials as LiCoO2, LiNiO2 and LiMn2O4.
Pouch type LiFePO4-natural graphite lithium-ion cells were cycled at constant current with periodic pulse-power testing in several different configurations. Components were analyzed after cycling with electrochemical, Raman and TEM techniques to determine capacity fade mechanisms. The cells with carbon-coated current collectors in the cathode and LiBOB-salt electrolyte showed the best performance stability. In many cases, iron species were detected on the anodes removed from cells with both TEM and Raman spectroscopy. The LiFePO4 electrodes showed unchanged capacity suggesting that the iron is migrating in small quantities and is acting as a catalyst to destabilize the anode SEI in these cells.
This paper reviews various chemical approaches that have participated in the improvement of the high temperature performance of the LiMn2O4/C Li-ion system. These approaches range from chemical surface and bulk modification of the spinel to the improvement of electrolyte stability towards acidification, and to the stabilization of the SEI chemistry of the carbon anode. More specifically, we describe the advantages of (1) modifying the surface chemistry of the spinel in order to obtain encapsulated particles or (2) modifying the crystal chemistry of the spinel through dual cationic and anionic substitutions by improving its stability towards Mn dissolution. The role of the carbon negative electrode towards the high temperature issue, namely through the formation/dissolution of the SEI layer is discussed, and a way of controlling such an SEI layer through a pre-conditioning of the cell is presented. The benefit of adding zeolites to the Li-ion cell to trap some of the species (H+, or others) generated during cell functioning as the result of the electrolyte decomposition or SEI layer is presented. Finally, from a compilation of other reports on that topic together with the present work, our present understanding of the failure mechanism in the LiMn2O4/C system is elucidated.